Determinants of Unloaded Shortening Velocity in Striated Muscle

Abstract

When a muscle is activated and shortens, the rate of shortening, the shortening velocity (v), varies inversely with the load (P) against which it shortens. This relationship is hyperbolic and is given by Hill’s classic equation (43):

$$ \left( {P + a} \right)v = b\left( {{P_0} - P} \right) $$

((1))

where P_o is the isometric force, and a and b are constants in units of P_o and muscle lengths/sec respectively. Typical values for frog skeletal muscle a and b at 0°C are 0.25 P_o and 0.33 l_o/s respectively. Total muscle fiber length, l_o, is usually reported at optimal thick and thin filament overlap (2.2–2.5 μm per sarcomere). This is important because shortening occurs by the relative sliding of the thick and thin filaments past each other in all the sarcomeres in the muscle; i.e., each sarcomere in series shortens about the same amount. Thus, the longer a muscle fiber or cell is (i.e., the greater the number of sarcomeres arranged in series), the faster the total length of the fiber or cell decreases for a given afterload. Maximal overlap for mammalian muscle cells occurs at about 2.4 μm/ sarcomere (thin filaments each are 1.1 μm long and the pseudo H-zone is 0.2μm long while the thick filament in all vertebrate striated muscle is about 1.65–1.7 μm). Thus if one sarcomere shortens against an afterload, P, at 2.4 μm/s, then 10 sarcomeres arranged in series shortening against the same load P, will shorten at a rate of 24 μm/s. Consequently the measured shortening velocity is referenced to the muscle fiber length at optimal overlap to normalize the observed values.